1. Field
The invention relates to a method for identifying gases and an associated device for identifying gases.
2. Description of Related Art
Methods of this type and the associated devices for detecting and identifying gases are used to recognise and identify chemical substances or compounds, in particular explosive materials or material compounds and/or those which are damaging to health, and which must be identified in very low concentrations.
The identification of explosive and/or toxic chemical compounds requires measuring methods with identification limits in the ppt-ppb range. In order to detect and identify these chemical compounds, spectrometers are therefore frequently used. Here, the use of ion mobility spectrometers (IMS), which are also known as plasma chromatographs, is to be preferred, since in contrast to other spectrometers such as a mass spectrometer, they are operated under atmospheric pressure and require no vacuum pump in order to generate an evacuated detection area. For this reason, IMS are small and low-cost with regard to their structural design compared to other spectrometers.
The area of application of the IMS is very wide. It ranges from the medical field, e.g. when examining the exhaled air of patients, through to use in production monitoring, e.g. when checking the quality of food products, and the military field, e.g. when identifying warfare agents. A general overview of IMS and their applications can be found for example in: G. A. Eiceman and Z. Karpas “Ion Mobility Spectrometry” (2nd edition, CRC, Boca Raton, 2005).
The structure and manner of functioning of the IMS are described in a large number of publications.
Thus, for example, in U.S. Pat. No. 3,621,240, a classic flight duration IMS is presented in which the different mobility of ions under atmospheric pressure is exploited. The target connections transferred into the IMS via an inlet system, e.g. a silicon membrane or a gas chromatography pillar, are continuously ionised in an ion source, by means of radioactive radiation, photoionisation or corona discharges. Very frequently, radioactive sources are used which directly ionise air molecules (nitrogen and oxygen). These ionised air molecules form the reactant ions H+[H2O]n and O2−[H2O]n. These reactant ions react with the interesting compounds by means of proton transfer, electron transfer or proton abstraction reactions, and form product ions MH+[H2O]n and MO2−[H2O]n. Depending on the concentration, dipole moment and humidity, dimers M2H+[H2O]n are formed under high target compound concentrations, or clusters are formed under a high residual humidity which have an increased number of water molecules n.
Within a very short period of time of approx. 200 microseconds and with the aid of an electric grid, these product ions are admitted into a drift tube which comprises an electric field and which accelerates the ions in a drift gas, usually filtered air under ambient pressure. It is commonly the case that the drift gas is guided in a pneumatically close gas circuit. This drift gas circuit contains elements, e.g. filters, which purify and condition the drift gas, since the state of the drift gas has a decisive influence over the detection capabilities. A molecular sieve is used as a filter, for example, with the aid of which the humidity of the drift gas is reduced until it reaches the level of a residual humidity in the lower ppm range. Continuous filtering is necessary since ambient humidity is constantly admitted into the IMS through the inlet system, albeit at a low level.
Due to the change in polarity of the electric field of the drift track, in a positive operating mode, positive ions can be identified, and in a negative operating mode, negative ions can be identified. Due to the electric field, the admitted product ions are constantly accelerated and constantly decelerated through impacts with the neutral molecules in the drift gas. Due to the electric field, the same tensile force acts on all ions with the same charge. Since however the product ions have different masses and impact profiles, they are characterized by different drift velocities. At the end of the drift tube, the product ions hit a detector at these different drift velocities. From the different flight times of the product ions through the drift tube, which typically lie within a range of 5 to 30 milliseconds, conclusions can be drawn regarding the chemical compounds to be studied. The drift velocity can be determined from the measured flight time or drift time and the known length of the drift track. With a lesser field strength E, e.g. E=200 V/cm, the drift velocity of the product ions vd is linearly dependent on the field strength. With these lower field strengths, the mobility K of the product ions can be expressed as follows, independently of the field strength:K=vd/E.
Since the measurements are conducted under atmospheric pressure, the drift velocity of the ions also depends on the temperature T, the pressure p and the residual humidity in the drift tube. In order to detect and identify the chemical compounds, the mobility of the product ions is always related to normal conditions, i.e. to a normal temperature T0=273° K. and a normal pressure p0=1013 hPa. Thus a temperature and pressure compensation takes place. The reduced or normalised mobility of the product ions can be shown as follows:K0=K·(T0/T)·(p/p0)=K·(273°K/T)·(p/1013 hPa).
However, a disadvantage with the use of the classic flying time IMS is that the residual humidity changes in the drift gas circuit. The residual humidity in the drift gas does however have a decisive influence over the detection capabilities. See also Mayer, Thomas; Borsdorf, Helko (2014): Accuracy of Ion Mobility Measurements Dependent on the Influence of Humidity. In: Anal. Chem. 86 (10), p. 5069-5076. The number n of water molecules directly influences the mass and impact profile of the cluster, its drift velocity and thus also its determined reduced mobility. With different residual humidities in the drift gas, a difference in the reduction in mobility can therefore also be anticipated. With product ions with a high level of mobility in particular, e.g. chloride ions clusters, this behaviour is clearly evident. Thus with the product ion Cl−[H2O]n with an average residual humidity below 1 ppm, reduced mobility of K0=2.80 cm2N/s can be anticipated, while with average residual humidities of 4 ppm, a reduced mobility of von K0=2.55 cm2/V/s can be anticipated. The reduced mobility of the product ion Cl−[H2O]n can thus also be regarded as a humidity indicator. Due to the different inclination of product ions to enter into clusters with water molecules, a deterioration in the differentiation of product ions can be anticipated. The probability of erroneous identifications increases. The use of residual humidity sensors here does not lead to the desired result, since on the one hand due to the low residual humidity in the lower ppm range only very expensive and large sensors such as dew point monitoring sensors can be used, while on the other, an automatic adjustment of the residual humidity is not possible.